Within the past few years, it has become increasingly recognized by orthopaedic surgeons that implant arthroplasty of the elbow is often necessary to restore function to and eliminate pain from elbow joints with severe surface damage. A significant factor in the increased treatment of diseased elbow joints by implant arthroplasty is the availability of well-engineered elbow joint prostheses that minimize failures of the implant and loosening of the cement-bone bond. Early designs provided virtually total constraint, thereby causing large loads to be exerted on the prosthesis at the limits of the range of articular motion and in inhibiting angulation in the varus-valgus plane and laxity in supination and pronation. The high loading produced a high incidence of implant failures and loss of bone-cement retention.
Presently available elbow joint prostheses are designed to permit limited laxity during supination, pronation and varus-valgus angulation, thereby allowing the tendons and muscles associated with the joint to absorb some of the loads and correspondingly reduce the loads imposed on the prosthesis at the limits of motion. One very successful elbow prosthesis developed at the Hospital for Special Surgery (the assignee of the present invention) and produced under license as the "Tri-axial.TM. Prosthesis" employs a unique "snap-fit" articulation in which a U-shaped plastic bushing attached by small pins to a metal humeral component accepts lateral and medial projections on a boss portion of an ulnar component within depressions in the legs of the bushing. The interaction of the bushing and the projections provides rotational and varus-valgus angular laxity in the prosthetic joint by permitting motions of the projections on the ulnar boss within the depressions of the humeral bushing, thereby absorbing energy and reducing loads imposed on the prosthesis.
While the "Tri-axial Prosthesis" has the very important advantages of virtually eliminating prosthesis failures and greatly reducing the possibility of loss of retention, a trade-off for these advantages is an increased likelihood of displacement of the joint--i.e., dislodgement of the ulnar boss from the humeral bushing.